Moraxella catarrhalis, also known as Moraxella (Branhamella) catarrhalis or Branhamella catarrhalis and formerly known as Neisseria catarrhalis or Micrococcus catarrhalis, is a gram-negative bacterium frequently found in the respiratory tract of humans. M. catarrhalis, originally thought to be a harmless commensal organism, is now recognized as an important pathogen in upper and lower respiratory tract infections in animals. In humans, M. catarrhalis causes serious lower respiratory tract infections in adults with chronic lung disease, systemic infections in immunocompromised patients, and otitis media and sinusitis in infants and children. See Helminen et al., 1993, Infect. Immun. 61:2003-2010; Catlin, B. W., 1990, Clin. Microbiol. Rev. 3:293-320; and references cited therein.
2.1. OUTER MEMBRANE PROTEINS AND PROTECTIVE ANTIBODIES
The outer surface components of Moraxella catarrhalis have been studied in attempts to understand the pathogenic process of M. catarrhalis infections and to develop useful therapeutic treatments and prophylactic measures against such infections. The outer membrane proteins (OMPs) in particular have received considerable attention as possible virulence factors and as potential vaccine antigens. M. catarrhalis has about 10 to 20 different OMPs with 6 to 8 of these, OMPs A to H, as the predominate species (Murphy and Loeb, 1989, Microbial Pathogen. 6:159-174). The molecular weights of OMPs A to H range from 97 to 20 kD, respectively. See Bartos and Murphy, 1988, J. Infect. Dis. 158:761-765; Helminen et al., 1993, Infect. Immun. 61:2003-2010; Murphy et al, 1993, Molecul. Microbiol. 10: 87-97; and Sarwar et al, 1992, Infect. Immun. 60:804-809. Comparisons of protein profiles by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) of outer membrane preparations from 50 M. catarrhalis strains show nearly homogeneous patterns of OMPs A to H (Bartos and Murphy, 1988, J. Infect. Dis. 158:761-765).
In addition to OMPs A to H, a high molecular weight OMP, designated HMW-OMP, having an apparent mass of 350 to 720 kD by SDS-PAGE has also been identified as another prominent surface component present in many strains of M. catarrhalis. HMW-OMP upon formic acid denaturation produces a single band of 120 to 140 kD and, thus, appears to be an oligomeric protein (Klingman and Murphy, 1994, Infect. Immun. 62:1150-1155). HMW-OMP appears to be the same protein as that designated UspA by Helminen et al., (1994, J. Infect. Dis. 170:867-872) and shown to be present in a number of M. catarrhalis strains.
In intact bacterium or bacterially-derived outer membrane vesicles, several of the above-identified OMPs present surface-exposed epitopes that elicit the production of antibodies that bind the OMPs. These antigenic OMPs include OMP E and OMP G (Murphy and Bartos, 1989, Infect. Immun. 57:2938-2941); OMP C/D (Sarwar et al., 1992, Infect. Immun. 60:804-809); CopB, an 80 kD OMP, (Helminen et al., 1993, Infect. Immun. 61:2003-2010); and UspA (Helminen et al., 1994, J. Infect. Dis. 170:867-872).
The therapeutic potential of antibodies to surfaced-exposed epitopes of CopB and UspA has been evaluated in an animal model. The model involved direct bolus inoculation of lungs of BALB/c VAF/Plus mice with a controlled number of M. catarrhalis cells and subsequent examination of the rate of pulmonary clearance of the bacteria (Unhanand et al., 1992, J. Infect. Dis. 165:644-650). Different clinical isolates of the M. catarrhalis exhibited different rates of clearance that correlated with the level of granulocyte recruitment into the infection site. Passive immunization with a monoclonal antibody directed to a surface-exposed epitope of either CopB or UspA increased the rate of pulmonary clearance of M. catarrhalis (Helminen et al., 1993, Infect. Immun. 61:2003-2010; Helminen et al., 1994, J. Infect. Dis. 170:867-872).
2.2. BACTERIAL/HOST CELL ADHERENCE AND HEMAGGLUTINATION
The adherence of bacterial pathogens to a host cell surface promotes colonization and initiates pathogenesis. See, E. H. Beachey, 1981, J. Infect. Dis. 143:325-345. Gram-negative bacteria typically express surface lectins that bind to specific oligosaccharides of glycoproteins and/or glycolipids on the host cell surface. Such lectins are often associated with pili or fimbriae. Bacterial adherence can also occur by non-specific binding resulting from hydrophobic and/or charge interaction with the host cell surface.
The mechanism of M. catarrhalis adherence to cells of the respiratory tract remains poorly understood. The organism adheres to cultured human oropharyngeal epithelial cells (Mbaki et al., 1987, Tohuku J. Exp. Med. 153:111-121). A study by Rikitomi et al. suggests that fimbriae may have a role in the adherence to such cells as fimbriae denaturation or treatment with anti-fimbriae antibodies reduced adherence by fimbriated strains (Rikitomi et al., 1991, Scand. J. Infect. Dis. 23:559-567). Fimbriae mediated binding, however, cannot be the sole basis of this adherence as the most highly adhering strain, among the several examined, was a non-fimbriated strain.
Hemagglutination reactions often replace the more complicated adherence assays in classifying bacterial adhesins. However, Rikitomi et al. found no correlation between human oropharyngeal epithelial cell adherence and hemagglutination by M. catarrhalis strains (Id.). That is three highly adhering strains did not agglutinate human erythrocytes. Thus, different binding mechanisms are involved in human oropharyngeal epithelial cell adherence and hemagglutination.
By contrast, a recent study by Kellens et al. suggests that hemagglutination by M. catarrhalis is correlated with host cell adherence (Kellens et al., 1995, Infection 23:37-41). However, this study employed an adherence assay based on bacterial binding to porcine tracheal sections. It is unclear whether porcine tracheal tissue can be considered homologous to human respiratory tract tissue with respect to adherence by pathogenic strains of M. catarrhalis.
Notwithstanding the problematic adherence assay, Kellens et al. examined the hemagglutination activities of eighty-some clinical isolates of M. catarrhalis (Kellens et al., 1995, Infection 23:37-41). Nearly three-quarters of the examined strains agglutinated human, rabbit, guinea pig, dog or rat erythrocytes, while the remaining strains did not. The agglutination activities for some of the hemagglutinating stains were further characterized and shown to be calcium ion dependent and inhibited by trypsin digestion or high-temperature treatment or addition of D-glucosamine or D-galactosamine.
A survey of hemagglutinating and non-hemagglutinating M. catarrhalis strains by Tucker et al. has shown that all strains bind the glycolipid gangliotetraosylceramide but only hemagglutinating strains bind the glycolipid globotetraosylceramide (Tucker et al., 1994, Annual Meeting of Amer. Soc. Microbiol., Abstract No. D124). Moreover, M. catarrhalis hemagglutination activity was shown to be inhibited by various monosaccharides that comprise the carbohydrate moiety of globotetraosylceramide. These observations led Tucker et al. to suggest that M. catarrhalis hemagglutinates by binding to globotetraosylceramides in the cell membranes of susceptible erythrocytes, including those of human red blood cells. To date, no prior art has identified a molecule on the outer surface of M. catarrhalis that is responsible for either host cell adherence or hemagglutination.
Citation or identification of any reference in this section or any other section of this application shall not be construed as an indication that such reference is available as prior art to the present invention.